1. Field of the Invention
The present invention relates to metal/polymer-resin bonded structured bodies and fabrication methods for such a structured body. The invention also relates to oxide coatings (formed on a metal base) for enhancing the bonding strength between the metal base and a polymer resin and forming methods for such a coating. The invention further relates to high reliability resin encapsulated semiconductor devices in which such an above-mentioned coating is provided between the encapsulating resin and the semiconductor device assembly, and fabrication methods for such a resin encapsulated semiconductor device.
2. Description of Related Art
In most semiconductor devices, a polymer resin needs to be bonded to metals such as Ni (nickel), Au (gold), Ag (silver), and Cu (copper). However, these metals have poor bondability to such a polymer resin and therefore can be a cause of reliability degradation of such semiconductor devices. In order to improve the bonding strength between a metal and a polymer resin, a method is disclosed in which an organic compound (such as a coupling agent or triazine) is applied over the surface of the metal (see, e.g., JP-A-2007-266562). Another method is disclosed in which a Zn (zinc) alloy coating having a mesh structure is formed on a metal surface by electroplating (see, e.g., JP-A-2007-508448, WO2005/021836). Still another method is disclosed in which the surface of Cu metal is oxidized in a hot alkaline solution to improve bonding strength (see, e.g., Mohamed Lebbai, et al.: J. Adhesion Sci. Technol., 17, 2003, pp. 1543-1560).
According to the JP-A-2007-266562, organic compounds such as coupling agents and triazine are coating materials containing both a functional group which can bond to metal and a functional group that can bond to organic materials. And, a crystallographically oriented layer made of such a coating material is formed on a metal surface, thereby allowing the metal to strongly bond to a polymer resin. A polymer resin can also be strongly bonded to insulators by choosing a suitable coating material.
According to the JP-A-2007-508448 (WO2005/021836), a coating made of Zn, Cr (chromium), or a Zn-alloy and having a mesh structure can be electroplated by dipping in an alkaline solution, and then a polymer resin infiltrates such a mesh structure to create an anchor effect by which the resin can strongly bond to the coating. Such a strong bonding strength can be sufficiently maintained even in harsh environments such as high humidity and severe thermal cycles, and therefore what is called a “popcorn phenomenon” which is caused by explosive vaporization of moisture accumulated in a semiconductor device can be prevented.
According to Lebbai et al., a Cu-oxide layer having a mesh structure can be formed on a Cu surface by oxidization in an alkaline solution, thus providing effects similar to those of the electroplating method of the JP-A-2007-508448 (WO2005/021836). However, the material usable for forming such a mesh structure by oxidation is limited to only Cu.
Conventional methods (such as those described above) for improving bondability by forming a coating using an organic material are effectively applicable to various underlying materials, but have a problem in that such a coating can degrade wettability of molten solders or degrade metal-to-metal bondability. In order to avoid this problem, such a coating needs to be selectively applied to only necessary areas of a surface of a semiconductor device. Or alternatively, unnecessary portions of such a coating (e.g., soldering portion, wire bonding portion and etc.) need to be removed after such the coating is applied onto a semiconductor device. However such procedures have a problem with degrading yield or productivity. The above procedures also have a manufacturability problem in that such selective coating is difficult at surface areas below bonding wires.
The above-described method of forming a coating having a mesh structure by electroplating can be applied to various underlying materials with electrical conductivity, but has a problem because amphoteric metals such as Al (aluminum) can be etched by the alkaline solution used for the plating. In particular, when such a surface treatment using an alkaline solution is applied to a semiconductor-device mounting assembly immediately before the process step of encapsulating a resin over the assembly, the Al thin film wirings of the semiconductor device can be damaged, leading to degraded circuit performance. This method also has the following limitations: (1) It is difficult to apply the method to an assembly in which some metal portions for electrical connection are disposed in floating islands. (2) An assembly to be electroplated needs to be sufficiently smaller so that it can be entirely immersed in the plating tank used. (3) The plating current density needs to be uniform over the surfaces of an assembly to be electroplated. Furthermore, in this method a cleaning procedure cannot be dispensed with, thus degrading productivity. Here, when such a cleaning procedure is insufficient, residues of the plating electrolyte used can cause corrosion in harsh use environments such as high temperature, high humidity, and biased conditions, thus potentially leading to degraded corrosion reliability.
The above-described oxidization treatment of Cu in a hot alkaline solution, similarly to the above electroplating method, has a problem in which the alkaline solution can etch and damage the Al wires for wire bonding or the Al thin film wirings of the semiconductor device. This method has another problem in that Cu in the bonding interface region is oxidized at high temperature and high humidity, and such a Cu-oxide layer can decrease the bonding strength. Further, similarly to the above-described electroplating method, this method also requires a cleaning procedure, thus degrading productivity.
Finally, in the above-described electroplating method, in order to avoid the problem of degraded reliability due to corrosion, a coating (which can strongly bond to a polymer resin) may be pre-electroplated to a bare lead frame by immersion in an electrolyte solution before a semiconductor device is mounted onto the lead frame. However, such an avoidance measure still has a problem because such a coating can degrade solder wettability and wire-bonding bondability (vital properties for semiconductor device assembly).